1. Field of the Invention
The present invention relates to hollow fiber membranes and methods of producing the same. In particular, the present invention relates to a method of producing a hollow fiber microfiltration membrane and hollow fiber ultrafiltration membrane to be used in water treatments, such as drainage treatments, water purification treatments, and industrial water production, and relates to a hollow fiber membrane produced by the method.
2. Description of the Related Art
Separation membranes such as microfiltration membranes and ultrafiltration membranes have been used in various fields such as the food industry, medical treatment, water production, and waste water treatment. In recent years, separation membranes have also been used in drinking water production, namely, water purification treatment. In the water treatment such as water purification, a large volume of water must be treated; hence, hollow fiber membranes having a large effective filtration area per unit volume are generally used. An improvement in the water permeability of the hollow fiber membrane allows a reduction in membrane area and a reduction in manufacturing expense due to the reduced size. Such an improvement is also advantageous since exchanging membranes becomes more cost effective and the membranes require a smaller installation area.
Fungicides such as sodium hypochlorite are added for sterilizing permeated water and preventing biofouling of the membrane in some cases. Furthermore, the membranes are washed with acids such as hydrochloric acid, citric acid, and oxalic acid, alkalis such as sodium hydroxide, and surfactants, if necessary. Hence, polyvinylidene fluoride separation membranes having high chemical resistance have been used recently. In water treatment, contamination by chlorine-resistant pathogenic microorganisms such as cryptosporidium has become common in the last few years. Under such circumstances, hollow fiber membranes must have high tensile properties to prevent contamination by raw water caused by fracture of the membranes. The term “raw water” represents river water, lake water, ground water, seawater, waste water, discharged water, and treated water thereof.
Polyvinylidene fluoride separation membranes are prepared by the following methods: (1) A polyvinylidene fluoride solution (polyvinylidene fluoride dissolved in a good solvent) is extruded from a spinneret or cast onto a glass plate held at a temperature that is considerably lower than the melting point of the polyvinylidene fluoride, and the shaped resin is brought into contact with a liquid containing a nonsolvent to form a porous structure by phase separation induced by the nonsolvent (wet process disclosed in Japanese Examined Patent Application Publication No. 1-22003); and (2) inorganic particles and an organic liquid are mixed with melted polyvinylidene fluoride, and the mixture is extruded from a spinneret or molded with a molding press held at a temperature that is higher than the melting point of the polyvinylidene fluoride, the resultant extrudate is solidified by cooling, then the organic liquid and the inorganic particles are removed to form a porous structure (melt extraction process disclosed in Japanese Patent No. 2899903).
The wet process, however, exhibits unevenness in phase separation in the thickness direction that causes the formation of a membrane having an asymmetric structure containing macrovoids; hence, the membrane has insufficient mechanical strength. Furthermore, there are many production parameters on which the structure and the properties of the membrane depend; the production steps are not controllable and reproducible. The melt extraction process yields a relatively uniform, high-strength membrane with no macrovoids; however, poor dispersion of the inorganic particles can cause defects such as pinholes. Furthermore, the melt extraction process has a disadvantage of extremely high production cost.